GEOCHEMICAL AND ISOTOPIC COMPOSITION OF APPALACHIAN BASIN BRINES: ORIGIN OF SALINITY AND FINGERPRINTS OF FLUID RESERVOIRS

Variable amounts of formation waters are often produced during shale gas extraction, which must be disposed of via deep injection or treated due to high salinities, trace metal concentrations and radioactivity. Characterizing the quality and quantity of in-situ shale formation waters is important for evaluating potential treatment and/or disposal options. In addition, it is important to characterize the geochemistry of shale formation waters in relation to overlying geologic formations and shallow aquifers, in order to identify sources of contamination in case of natural or drilling-induced brine migration. This paper provides an overview of Appalachian Basin brine geochemistry, focusing primarily on the origin of salinity, and geochemical and isotopic fingerprints of Devonian Marcellus Shale formation waters in relation to fluids in other geologic formations.

Produced waters from Marcellus Shale gas wells, after injection of low-TDS hydraulic fracturing fluids and subsequent flowback, are highly saline (14 to 248 g/L TDS). These brines are dominated by Na-Ca-Cl and contain elevated Br concentrations and Br/Cl ratios, indicative of paleoseawater evaporated past halite saturation and further modified by subsequent diagenetic reactions. Variable solute concentrations with relatively constant solute to bromide ratios suggest later brine dilution by freshwater. Oxygen and hydrogen isotopes plot to the right of the Global Meteoric Water Line, along a mixing trend between evapoconcentrated seawater and a meteoric water endmember. Marcellus Shale formation waters have similar major ion chemistry and water stable isotope compositions to adjacent Upper Devonian shales and sandstone formation waters, yet are distinct from brines in underlying Silurian formations and dilute groundwater in shallow aquifers. Strontium isotope ratios of Marcellus Shale brines are less radiogenic than formation waters in overlying shale and sandstone formations. In addition, produced waters from the Marcellus Shale are highly radioactive, with median total radium concentrations greater than non-Marcellus formations and fluids in other sedimentary basins worldwide.